In the production of die cast rotors for electric motors, it is common to position a stack of steel rotor laminations between a set of molds which define cavities for producing end rings on opposite ends of the lamination stack. The stack of laminations are secured together, for example, by welding or a pressed-in arbor or by interlocking adjacent laminations at the stamping press. Each mold may define a single cavity for producing a single die cast rotor or may define multiple cavities for simultaneously producing a plurality of rotors. The laminations within each stack are provided with peripherally spaced openings or slots through which die cast material flows from one end ring mold to the opposite end ring mold for casting internal bars which integrally connect the cast end rings. When the rotor laminations have peripherally spaced slots extending to the outer surface of the lamination stack, a cylindrical casting mold or retainer surrounds the lamination stack to confine the die cast material as it is forced through the slots to form the end rings and connecting bars.
After the assembly of the lamination stack, surrounding retainer and end ring molds is clamped within a die cast machine or press, the preheated die casting material, which is commonly aluminum, is injected into one of the molds for filling the end ring cavities within the molds and for filling the circumferentially spaced slots within the lamination stack. After the cast aluminum solidifies in response to a flow of cooling water through holes within the molds, the press is opened and the aluminum die cast rotor is removed from the press.
Preferably, die cast rotors are produced in a vertical die cast press, for example, of the type manufactured by THT Presses Inc., the assignee of the present invention. When this vertical die cast press is used to produce aluminum die cast rotors, an electrical resistance furnace is used to heat a volume or vat of aluminum sufficient to produce a large number of aluminum die cast rotors. During each cycle of the press, an automatic ladle dips down into the vat of molten aluminum and transfers a batch of aluminum to a cavity defined within the top portion of a cylindrical shot sleeve enclosing a vertically movable and hydraulically actuated shot piston.
After the ladle is removed from the press, the lamination stack and the lower end ring mold are moved into the press by a shuttle, and the upper end ring mold is moved downwardly by the press on top of the stack to clamp the assembly of the molds and stack against the shot sleeve. The molten aluminum is then forced upwardly through gates within the lower end ring mold by upward movement of the shot piston. The molten aluminum flows upwardly into and through the end ring cavity within the lower mold, through the bar slots within the lamination stack and into the end ring cavity within the upper mold. After the aluminum has solidified, the press is opened which raises the upper end ring mold. The lower end ring mold and die cast rotor are then shifted laterally out of the press by the shuttle to a position where the die cast rotor is ejected upwardly from the lower mold by a fluid cylinder.
When a rotor is die cast with aluminum, the electrical conductivity of the rotor cage is usually no more than 62% IACS (International Annealed Copper Standards). This limits the current density within the aluminum conductor bars extending within the lamination slots and connecting the die cast end rings. This electrical conductivity limitation affects conductor bar size, maximum conductor bar current due to heating and overall rotor efficiency.
It is well known that if a die cast rotor could be produced using copper in place of aluminum, the higher conductivity of copper could produce a more efficient rotor for an electric motor. For example, U.S. Pat. Nos. 2,607,969 and 2,991,518 mention that die cast rotors may be produced with a conducting metal such as copper, aluminum, etc. However, these patents do not recognize the unusual problems associated with die casting copper to form a rotor for an electric motor. For example, copper has a melting temperature of about 1980.degree. F. as compared to about 1220.degree. F. for aluminum. Copper also has a density of about three times greater than aluminum and rapidly absorbs oxygen and hydrogen which reduce conductivity and produce unacceptable cracking in a copper cast rotor.